1. Field of the Invention
This invention relates to processes and apparatus for plating metals onto a workpiece and more particularly to processes and apparatus for relatively uniformly, and selectively plating of small features on a workpiece.
2. Description of Related Art
A serious problem in electroplating microscopic features non-uniformly dispersed on a large substrate of a workpiece is that the total current required for depositing the material onto these microscopic features is very small. There are no commercially available power supplies that can reliably deliver the required kind of small currents to the workpiece/substrate.
FIGS. 1A-1C are schematic diagrams which illustrate a prior art type of plating system using a thief electrode ring 22 surrounding a workpiece 26 such as a silicon wafer to enhance the quality of plating of metal onto the workpiece 26 through a photoresist mask formed thereon (not shown) as is widely practiced in the art. In FIG. 1A which is a vertical elevational view, a plating tank 10 has a bottom 12, left sidewall 14 and right sidewall 15 and a top 16 shown to be open. FIG. 1B shows the thief ring 22 and the substrate 26 isolated from the other elements seen in FIG. 1A. FIG. 1C is a right side view of what is shown in FIG. 1B. In the example shown here, the tank 10, which is formed of a dielectric material, contains an electroplating bath 17 up to the level shown by line 18. An anode 33 is located in the plating tank 10 near the right sidewall 15. A positive voltage V3 is applied to anode 33 by a connection wire 32. The workpiece 26, which is shown on the side of the tank 10 near the left sidewall 14 has a negative voltage (xe2x88x92V1) applied thereto by a connection wire 36. There is a thief ring 22, which surrounds the workpiece 26 is coplanar with the workpiece 26. A second negative voltage (xe2x88x92V2) is applied to the thief ring 22 by a connection wire 34. A space 24 is provided between the workpiece 26 and the thief ring 22. The thief ring is adjusted in voltage to adjust the plating current to the workpiece 26, but it is not possible to maintain an equal current density across the large surface of the substrate 26 which may be several inches wide.
As a result, the thickness of the material deposited on various features on the workpiece 26 can vary from workpiece to workpiece. This variation creates a very big quality control problem for the plating engineer who is required to deposit a desired thickness on all of the features of the workpieces 26 within narrow tolerances.
A second and more important problem in dealing with small features dispersed on a large substrate 26 is that the secondary current, and higher order currents, cause tremendous non-uniformity in the thickness of the deposited material from place to place across the workpiece 26. This non-uniformity will vary depending upon the density of the features and also on the size of the features on the substrate 26.
Prior art relating to cathodes in electroplating baths include the following patents.
U.S. Pat. No. 3,652,442 of Powers et al. for xe2x80x9cElectroplating Cell Including Means to. Agitate Electrolyte in a Laminar Flowxe2x80x9d describes a Horizontal Paddle Electroplating Cell (HPEC) in which a cathode in the form of an insulating board to which is affixed a conductive sheet or coating with a very smooth upper surface. The cathode is shown with its flat upper surface extending horizontally at the bottom of the cell lying on a conductive support block. The bath is agitated during plating by a base portion which moves continuously at a substantially uniform rate in a path back and forth along the length of the cathode and just above the surface thereof. The result is that the bath solution is homogenized on the surface of the cathode. Agitating means is provided including a motor connected by linkages to the base portion which causes a uniform laminar flow of the bath across the surface of the cathode without causing any measurable turbulence on the surface thereof. The agitating base, which is designed to cause minimal resistance to flow of the bath, is triangular in cross section with a blunted apex at an angle which permits flow thereover with minimal turbulence, while at its base which confronts the cathode the agitating base is flat so that the agitating caused by the agitating base caused the bath to flow over the base and to effect mixing with the bulk of the bath at the apex of the base by convection. As the mixture passes the apex, the laminar flow is restored. The system is used to plate magnetic metal alloys.
In U.S. Pat. No. 4,102,756 of Castellani et al. entitled xe2x80x9cNickel-Iron (80:20) Alloy Thin Film Electroplating Method and Electrochemical Treatment and Plating Apparatusxe2x80x9d, which describes another HPEC for plating films to form batch-fabricated, magnetic bubble devices and magnetic recording thin film heads, in which the plating bath is maintained at a level at which the anode is immersed in the bath during electroplating of a magnetic recording device. The constituents of the bath are constantly replenished and bath temperature is controlled by recirculation from a reservoir where it is refreshed by dispensing acid, iron and preferably also Na, Saccharin, Na lauryl sulfate and/or Ni++ if needed and constantly stirred by a horizontal reciprocating mixer otherwise referred to herein as a paddle, which travels back and forth horizontally above the surface of the cathode at an approximate distance of {fraction (1/32)} to xe2x85x9 inch (79 mm to 318 mm) for providing agitation of the bath with minimal turbulence.
U.S. Pat. No. 5,516,412 of Andricacos et al. describes a xe2x80x9cVertical Paddle Plating Cellxe2x80x9d (VPPC) which is a modification of the Castellani et al cell adapted for microplating metal onto a substrate an article which is a flat, circular wafer or substrate having a substantial number of individual IC chip patterns arranged suitably thereon. The microplating process may comprise electroplating or electroless plating process. As microplating techniques were being developed for manufacturing devices such as features with a trend to continuously smaller and smaller dimensions of integrated circuits (ICs) in the form of microscopic chips formed on a flat circular wafer or substrate, it became necessary to reorient the plating system to suspend the article being plated vertically to remove debris from the surface being plated. Furthermore, as metal ions are depleted from the electrolyte, the uniformity of the electrolyte is decreased and must be suitably corrected to avoid degradation of the electroplating process so use of the laminar flow type of paddles was required to assure uniformity of the composition of the plating bath at the microsufaces being microplated. Because of the very small areas being plated in the microplating process of forming microcircuits on IC devices, a thief electrode was added behind the article being plated extending beyond the periphery thereof to enhance performance. Moreover the clearance between the surface of the article (substrate) being plated and the laminar paddle was decreased by one or more orders of magnitude to 1 mm to 4 mm from the 79 mm to 318 mm of the above HPEC plating apparatus of Castellani et al., U.S. Pat. No. 4,102,756. Thus the plating cell was adapted for electroplating the exposed surface of an article that is supported vertically on a vertical rack. The rack includes a thief electrode laterally surrounding the article to define a cathode. The cell includes a reciprocating vertical paddle (of the kind described in the above Powers et al. patent) which includes two elongated, parallel prisms which have oppositely facing, parallel, flat bases with one of the bases being disposed parallel to and closely adjacent to the article or rack for parallel movement over the article supported therein, preferably skimming across the surface of the article being plated, about 4.0 mm therefrom. Since the surface of the article to be microplated is preferably disposed vertically, and relative to gravity, the VPPC includes an elongated paddle which is disposed vertically lengthwise in the plating cell adjacent to the article being plated and rack. Means are provided for reciprocating the paddle between the front and back walls of the plating cell for suitably agitating the electrolyte inside the cell to diminish adverse plating effects from buoyancy or gravity induced convection within the plating cell. The reciprocating paddle is in the exemplary form of a pair of vertically elongated, triangular (45xc2x0xe2x88x9290xc2x0xe2x88x9245xc2x0) prisms having spaced apart, parallel apexes defining a throat therebetween through which the electrolyte flows. Suitable means are provided for bathing or filling a cell and an outer cell with electrolyte to the desired elevation above the inner cell for providing overflow discharge from an outlet weir to continuously recirculate the electrolyte through the inner cell, as well as through the outer cell. A suitable external reservoir is provided suitably remote from the VPPC for storing as well as providing a suitable source of the electrolyte. One or more suitable flow conduits join the outlet trough, the reservoir, and the inner cell in a closed-loop fluid circuit for recirculating the electrolyte. A suitable pump is disposed in the flow conduit between the inner cell and the reservoir for continuously recirculating the electrolyte in the fluid circuit. A suitable filter is also disposed in the flow conduit between the pump and the inner cell for filtering the electrolyte prior to return thereof to the inner cell. Suitable temperature control of the electrolyte is typically also provided for providing suitably clean electrolyte at the preferred temperature in a conventional manner.
Currently, where the article to be plated is a semiconductor wafer upon which microcircuits are being formed, non-uniformity of microplating is a problem caused by the very low density of the area of the metallic surfaces as a percentage of the pattern design. In addition, the clearance between the wafer and the reciprocating paddle in such a system is in the order of 1-5 mm. When the metallic areas of the wafer design feature density are very low ( less than 1%) or very patchy (localized here and there) the thief used in the prior art can no longer function properly. The problem being encountered is that substantially all of the plating current is drawn to the thief due to its dominant size because the density of the plating surface of the thief approaches 100% vs. the density of plating surface in the wafer which may be as low as 1% or less. Thus there is a need for a solution to the problem of non-uniformity of plating to achieve proper functioning of the plating system by assuring that enough of the ions approaching the cathode are directed towards the article to be microplated.
U.S. Pat. No. 6,027,631 of Broadbent for xe2x80x9cElectroplating System with Shields or Varying Thickness Profile of Deposited Layerxe2x80x9d, which is concerned with plating a blanket layer across a substrate, describes an electroplating system where a shield is placed above and adjacent to a workpiece; and the workpiece is rotated, so as to form uniform plating across the workpiece. The process described employs physical obstruction of current by use of the shield(s). However, the shield(s) is electrically inactive and is inserted between cathode (part or substrate) and the anode. The shield is placed in location and the substrate is rotated with respect to the shield. Thus the substrate or wafer is exposed to the anode part of the time. It is believed by the inventors that this method is applicable to plating larger features in inert matrix photoresist mask, but not to plating of small features distributed in a non-uniform distribution across the entire substrate. Also this method does not help when the active area that needs to be plated is small since the shield is not electrically connected to the substrate to increase the xe2x80x98apparent sizexe2x80x99 of the workpiece or substrate.
U.S. Pat. No. 6,077,405 of Biggs et al., commonly assigned, for xe2x80x9cMethod and Apparatus for Making Electrical Contact to a Substrate During Electroplatingxe2x80x9d also shows a peripheral ring electrode, often referred to as a xe2x80x9cthief ringxe2x80x9d since it is an auxiliary cathode which diverts cathode current away from the primary cathode. The Biggs et al. patent describes the structure of exemplary substrates and mechanical and electrical connections to the substrates.
U.S. Pat. No. 5,135,636 of Yee et al. for xe2x80x9cElectroplating Methodxe2x80x9d describes a plating rack for use in electroplating at least one substrate comprising a silicon wafer surrounded by a metal ring with cam assemblies holding the wafer in place and for making electrical contact between the ring and the wafer and passing a current from the ring to the wafer while they are submerged in an electroplating bath.
U.S. Pat. No. 5,620,581 of Ang for xe2x80x9cApparatus for Electroplating Metal Films Including a Cathode Ring, Insulator Ring, and Thief Ringxe2x80x9d describes apparatus for electroplating metal films composed of dual metal, i.e. a PERMALLOY(trademark) type of (NiFe) alloy, where a wafer workpiece is set inside a thief ring and coplanar to the ring. The part is connected to a first power supply. The power to the thief ring is described by text which is at variance with the drawings which show a second connection line to a common connection to a D.C. voltage source which is referred to as a xe2x80x9cdual channel power supply . . . employed to generate separately controlled current densities to the thief ring . . . and the cathode ring . . . xe2x80x9d which is used so that by controlling xe2x80x9cthe thief current density, the metal composition of the electroplated metal film is controlled.xe2x80x9d There is also a stainless steel xe2x80x9ccathode ringxe2x80x9d which mechanically supports the lower surface of the substrate/wafer which electrically connects the wafer to a power supply. The objectives of Ang include a compositional uniformity as well as thickness uniformity. Essentially, the Ang patent addresses edge effects and the primary current distribution problem.
U.S. Pat. No. 6,001,235 of Arken et al., commonly assigned, for xe2x80x9cRotary Plater with Radially Distribute Plating Solutionxe2x80x9d shows a rotating cathode and a rotating segmented ring formed of a set of separated annular thief elements.
U.S. Pat. No. 6,071,388 of Uzoh for xe2x80x9cElectroplating Workpiece Fixture Having Liquid Gap Spacerxe2x80x9d shows a peripheral thief ring electrode. Uzoh suggests that the thief ring electrode should comprise of a stainless steel or titanium plate including a metal mesh or screen such as No. 4 or No. 30 metal mesh corresponding to wires per inch. The mesh increases the surface area of the thief ring electrode, but does not solve the problem of non-uniformity of plating of small features.
U.S. Pat. No. 6,074,544 of Reid et al. for xe2x80x9cMethod for Electroplating Semiconductor Wafer Using Variable Currents and Mass Transfer to Obtain Uniform Plated Layerxe2x80x9d describes forming a metal seed layer and providing electrical contacts at the edge of a wafer which leads to the dish-effect in which the thickness of the layer is less in the center. Reid teaches minimization of the dishing effect by using a low plating current density initially to reduce the resistive (IR) voltage drop followed by increasing the current density to a higher level after reaching a predetermined thickness and resistivity.
U.S. Pat. No. 4,828,653 of Traini et al. for xe2x80x9cLong Lasting Anode for High Current Density Galvanizationxe2x80x9d relates to anodes in electroplating baths includes the following patent, which is not analogous to this invention since it pertains to cathodes in electroplating baths includes the following patent but does show to employment of a mesh in a plating bath electrode well over a decade ago. Traini et al. describes a long lasting anode formed by several parallel layers of foraminous (i.e. having small openings or perforations) sheets of metallic mesh with different patterns. The sheets of metallic mesh are resistant to the electrolyte such as Ti, Ta, NB or W in electrical contact with each other. The metals used in the mesh are preferably inert to a plating liquid in a electroplating bath presumably to prevent dissolution of the metals in the mesh into the plating solution during plating of the cathode.
The current electroplating process used in some thin film applications such as semiconductor packaging uses a fixture that holds a workpiece/substrate and an auxiliary electrode also widely known as a thief plate. The auxiliary (thief) electrode surrounds the actual workpiece in such a way that the substrate surface and the auxiliary electrode surface are in a plane. The main workpiece and the auxiliary electrode can be connected to two different power supplies so that the voltages/currents can be controlled independently. This arrangement works fairly well when the active area being plated is relatively large and uniformly distributed across the entire substrate. However, problems occur with the peripheral thief ring process when the active area on the substrate is microscopic and/or when the area is non-uniformly distributed over the substrate surface.
There are problems with electroplating microscopic features which are often out of the primary current distribution region. There is a lack of sufficient thieving activity to provide the desired secondary current distribution and higher order current distribution in the plating bath which will permit uniform plating. Moreover there is a lack of commercial power supplies that can reliably deliver small currents to electroplate small areas. In addition there has been an inability to provide methods and means for electroplating myriad microscopic features with unknown active areas. In the ever increasing trend towards smaller and smaller microscopic electronic devices it is not possible to provide apparatus and methods which can be tailored to each permutation of distribution of microscopic features on a workpiece.
In addition there is a limitation of substrate size that can be used with traditional peripheral thieving rings surrounding the workpiece.
It is an object of this invention to electroplate microscopic features that are non-uniformly dispersed on a large substrate.
An object of this invention is an electroplating process including plating fixture for electroplating very microscopic features dispersed on a large substrate with a high degree of uniformity.
A process in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate having a surface to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a portion thereof with openings therethrough extending across the plating surface of the substrate and positioned between the substrate and the anode. Connect power to provide a negative voltage to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.
Preferably, the openings comprise apertures through the second cathode extending across the substrate the openings are formed between parallel wires extending across the substrate, or the openings comprise a wire mesh extending across the substrate, or the openings comprise apertures formed in a plate which is preferably a stainless steel plate located between the anode and the cathode.
Preferably, the first cathode and the second cathode are connected to an adjustable power supply.
Preferably, the first cathode and the second cathode are connected to the same power supply.
Preferably, the first cathode and the second cathode are connected to the same power supply with a resistor in series with the second cathode.
In accordance with another aspect of this invention, a process for electroplating a film onto a substrate having a top side including a plating surface comprises the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a partially open screening electrode selected from a plating mesh and aperture plate and parallel wires proximate to the substrate between the substrate and the anode. Connect a power supply by providing a negative voltage to the first cathode and to the screening electrode and providing a positive voltage to the anode.
The openings comprise a wire mesh extending across the substrate and the first, cathode and the second cathode are connected to power supplied in a manner selected from the group consisting of a power supply with a resistor in series with the second cathode, the first cathode and the second cathode are connected to the same power supply, the first cathode and the second cathode are connected to same power supply with a resistor in series with the second cathode, and the first cathode and the second cathode are connected to an adjustable power supply process in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. Provide a plating tank with an electroplating bath. Provide an anode in the bath. Place a substrate having a plating surface to be electroplated into the electroplating bath connecting surfaces to be plated to a first cathode. Support a second cathode including a portion thereof with openings therethrough extending across the plating surface of the substrate and positioned between the substrate and the anode. Connecting power to provide a negative voltage to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.
Apparatus in accordance with this invention is provided for electroplating a film onto a substrate having a top side including a plating surface includes the following steps. The apparatus includes a plating tank with an electroplating bath, an anode in the bath. A substrate having a plating surface to be electroplated is placed in the electroplating bath with surfaces to be plated connected to a first cathode. A second cathode including a portion thereof with openings therethrough is supported extending across the plating surface of the substrate and positioned between the substrate and the anode. Power to provide a negative voltage is connected to the first cathode and provide a negative voltage to the second cathode, and provide a positive voltage to the anode.
Preferably, the openings comprise apertures through the second cathode extending across the substrate the openings are formed between parallel wires extending across the substrate, or the openings comprise a wire mesh extending across the substrate, or the openings comprise apertures formed in a plate which is preferably a stainless steel plate located between the anode and the cathode.
Preferably, the first cathode and the second cathode are connected to an adjustable power supply, the first cathode and the second cathode are connected to the same power supply, and the first cathode and the second cathode are connected to the same power supply with a resistor in series with the second cathode.
In accordance with another aspect of this invention, means are provided for electroplating a film onto a substrate having a top side including a plating surface comprises the following steps including a plating tank with an electroplating bath and an anode in the bath. The substrate to be electroplated is placed into the electroplating bath connecting surfaces to be plated to a first cathode. A second cathode is supported in the bath. The second cathode includes a partially open screening electrode selected from a plating mesh and aperture plate and parallel wires proximate to the substrate between the substrate and the anode. A power supply is connected by providing a negative voltage to the first cathode and to the screening electrode and providing a positive voltage to the anode.
The openings comprise a wire mesh extending across the substrate and the first cathode and the second cathode are connected to power supplied in a manner selected from the group consisting of a power supply with a resistor in series with the second cathode, the first cathode and the second cathode are connected to the same power supply, the first cathode and the second cathode are connected to same power supply with a resistor in means for providing, and the first cathode and the second cathode are connected to an adjustable power supply.